tools: ynl-gen: mutli-attr: support binary types with struct

Binary types with struct are fixed size, relatively easy to
handle for multi attr. Declare the member as a pointer.
Count the members, allocate an array, copy in the data.
Allow the netlink attr to be smaller or larger than our view
of the struct in case the build headers are newer or older
than the running kernel.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250429154704.2613851-9-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>